Answer:
External force on him will be 112 N
Explanation:
We have given the mass of the sprinter m =70 kg
Acceleration of the sprinter 
We have to find the net external force
According to second law of motion force = mass ×acceleration
Force is dependent on the mass and acceleration
So 
So external force will be 112 N
The answer you are looking for is B, hope this helps.
Answer:
Accelerating charges.
Explanation:
Electromagnetic waves are waves produced by the vibration of both electrical and magnetic fields.
This interaction produces an energy source that does not require any medium to propagate.
To produce electromagnetic waves, electric and magnetic fields must be vibrating.
An electric charge produced when vibrating under voltage will produce electromagnetic waves. This is the same for all sources of these waves.
The sun produces electromagnetic waves. A lot of human activities also does this.